This invention relates to the formation of an electrically conductive contact on a substrate (e.g. printed circuit board, semiconductor chip and chip carrier).
Processes for integration of semiconductor chips and other components onto circuitized carriers rely on a bonding method to join the electrodes of the chip or component to a substrate. This can be achieved by metallurgical bonding such as soldering, or by use of a conductive adhesive. Various methods can be used to form the contacts on chips, electrical components, and on substrates. In one example a solder ball preform of a high melt solder is fabricated first and then bonded to a component or substrate with a lower melt alloy. High melt solder bumps have also been formed by evaporation techniques such as on semiconductor chips. Component bump formation and solder ball bonding are usually processed en masse, in an evaporator or oven to cause melting. Formation of high temperature bumps on organic substrates can become quite problematic because of the potential damage that can be caused by subjecting the substrate or component to the elevated temperature required to form the bump. The equipment and processing associated with these technologies are relatively expensive and difficult to control for reliable bump or joint formation. More specific examples of various methods for forming solder-type contacts are discussed in the following U.S. Patents and publication.
U.S. Pat. No. 5,586,715 by Schwiebert et al., a method is described to produce solder balls by contained paste deposition using an attached mask on non-wettable substrate, and heating the substrate-mask assembly at a temperature to reflow the solder paste into a solder ball. The invention is limited in that it relies on having to use dewettable components to form a ball and heating the entire substrate, mask, and solder material en mass.
In U.S. Pat. No. 5,539,153 by Schwiebert et at., a method is described for bumping substrates by contained paste deposition using an attached mask on substrate. The invention is limited in that the entire substrate, mask, and solder paste are all heated to the solder reflow temperature to form the resultant solder bump and is dependent on having a non-wettable mask to contain the solder paste.
In U.S. Pat. No. 5,658,827 by Aulicino et al., a method is described for forming solder ball contacts on substrates by squeegeeing solder paste through apertures in reusable mask. The method describes heating the reusable mask containing the solder and the substrate to reflow temperature in order to form the resultant balls.
U.S. Pat. No. 4,229,232 by Kirkpatrick, describes a method for thermal processing on or near the surface of a metallic or dielectric material using a pulsed beam. The method is limited in that it thermally treats only the selected area""s surface or near surface, rather than heating and melting the entire solder paste layer thickness and additionally heating the circuitized feature to form a solder bump bonded to the circuited feature.
U.S. Pat. No. 4,832,982 by Mori et al., describes a process for forming a dispersion alloy layer on a metallic base starting with a powder alloy which separates into two separate liquid phases when irradiated by a laser and is then quenched to form a solid on a substrate surface. The process requires the formation of two liquid phases over a substrate surface rather than forming a solder bump on a circuitized feature.
U.S. Pat. No. 5,509,597 by Laferriere, describes a process and apparatus wherein solder paste is applied over a first material, a second material is placed over the solder paste and the structure is heated by a laser beam to produce a solder joint between two materials. The apparatus and method are limited in that these are concerned with soldering two materials together rather than forming solder bumps on circuitized features for subsequent soldering.
U.S. Pat. No. 5,272,307 by Jones, similarly describes a method and apparatus to solder leads together with heat from a laser. A work plate is used to hold the two leads together while the solder is melted between the two leads. This patent does not describe forming solder bumps on circuitized features.
U.S. Pat. No. 5,156,697 by Bourell et al, describes a method and apparatus to direct a laser onto a powder comprised of a low temperature material and a high temperature material to produce a bulk sintered mass. Here the lower temperature material melts while the higher temperature remains unmelted while forming the sintered mass. This patent relies on two materials of differing melting temperatures and attempts to form bulk material compounds and not solder bumps on a circuitized feature.
U.S. Pat. No. 5,641,113 by Somaki et al., describes a process where a solder ball is reflowed over an electrode on a substrate to form a solder bump. Then, another ball of similar metallurgy is placed on top of the solder bump and reflowed to form an interconnection joint. The process requires a stencil to contain the first reflowed solder ball and relies on the same ball metallurgy for joining the second ball to the first ball.
U.S. Pat. No. 4,865,245 by Schulte et al., describes a method for joining two semiconductor devices by applying pressure to two devices to cause a cold weld junction between respective solder bumps of each semiconductor. The method relies strictly on the ability to cold weld between two electrode bumps and requires applying potentially damaging pressure to the semiconductor devices which could cause crack initiation.
U.S. Pat. No. 3,836,745 by Costello, describes a method of solder coating and joining members by heating a predetermined portion of members through the use of radiant energy adsorbent material deposited on the members. The method is limited in that it requires the heating of the entire ribbon layer of solder paste between members and over members, relying on wicking away of the molten solder between the members to prevent solder bridging.
IBM Technical Disclosure Bulletin, Vol. 38, No. 05, May 1995, describes an additive that is added to solder paste to improve the adsorption characteristic of the solder paste to reduce the energy required of a laser to cause solder reflow.
This publication does not describe formation of solder bumps on circuitized features using laser or focused infrared energy source.
It is believed that the method of selective heating of a layer of solder paste over a circuitized feature having the advantageous features cited herein and otherwise discernible from the aforementioned teachings would represent a significant advancement in the art.
It is a primary object of this invention to enhance the art of providing electrical contacts on circuitized substrates.
It is a further object to provide a method of making such contacts wherein solder forms an important part thereof.
It is yet another object of the invention to provide a method for selectively heating an area of solder paste over a circuitized feature to form a solder bump which can be used for a subsequent interconnection to contacts of another component or components.
According to one aspect of the invention, there is provided a method of making at least one electrically conductive contact on a substrate comprising the steps of: providing a substrate with a surface having at least one circuitized feature; applying a layer of solder paste over the surface of the substrate and at least a portion of the circuitized feature; providing a mask having a top surface and a bottom surface and at least one opening; orienting the mask such that at least one opening in the mask is aligned with the circuitized feature; positioning the bottom surface of the mask on the layer of the solder paste; directing a focussed radiation beam having a beam diameter larger than said mask opening on said solder paste through said opening in said mask and selectively heating substantially only the solder paste over the circuitized feature through the mask sufficiently to melt the solder paste over the circuitized feature to form a substantially solid conductive solder bump while not significantly heating the remainder of the solder paste over the surface of the substrate or substantially altering the configuration; removing the mask and removing the solder paste that was not significantly heated.
Another aspect of the invention is a method of making at least one electrically conductive contact on a substrate comprising the steps of: providing a substrate with a surface having at least one circuitized feature; applying a layer of solder paste over the surface of the substrate and at least a portion of the circuitized feature; providing a mask having a top surface and a bottom surface and at least one opening; orienting the mask such that at least one opening in the mask is aligned with the circuitized feature; positioning the mask immediately above the layer of solder paste at a spaced distance; directing a focussed radiation beam having a beam diameter larger than said mask opening on said solder paste through said opening in said mask and selectively heating substantially only the solder paste over the circuitized feature through the mask sufficiently to melt the solder paste over the circuitized feature to form a substantially solid conductive solder bump while not significantly heating the remainder of the solder paste or substantially altering the configuration, the heating and forming of the conductive solder bump occurring without the mask contacting the solder paste or substrate; removing the mask; and thereafter removing the solder paste that was not significantly heated.